The Emerging Threat of the Polyphagous Shot Hole Borer (PSHB): A Tree-Killing Invasive Beetle
The Polyphagous Shot Hole Borer (PSHB), is a minuscule beetle is already wreaking havoc on trees worldwide. It’s an invasive species from Southeast Asia, and is no ordinary pest. Measuring just 2-3 millimeters long, it bores into tree trunks and branches, creating galleries where it cultivates symbiotic fungi. These fungi, including species like Fusarium euwallaceae, block the tree’s vascular system, cutting off water and nutrients, leading to branch dieback and eventual tree death. The result? Devastated forests, urban and suburban landscapes, agricultural losses, and disrupted ecosystems. It won’t be eradicated, and it will eventually cost a fortune to control.

What makes the PSHB such a formidable threat? Its “polyphagous” nature means it attacks a staggering variety of hosts — over 200 tree species, from oaks to maples, from avocados and citrus, to willows and even castor beans.
Unlike many borers that target stressed trees, PSHB can infest healthy ones, spreading rapidly through infested wood movement, like firewood or green waste.
First noted in California in 2003, it has since invaded regions in Israel, South Africa, and Australia, causing millions in economic damage. For homeowners, farmers, and environmentalists, understanding this pest is crucial to safeguarding our green spaces.
Infections Rampant in Southern California
Southern California has been ground zero for PSHB in the U.S. The beetle was first detected in 2003, but it wasn’t until 2012 that its destructive partnership with Fusarium fungi became evident in Los Angeles and Orange Counties, initially hitting avocado groves hard. By early 2023, infestations had spread to seven counties, including San Diego and Riverside, affecting urban trees like sycamores, box elders, and oaks. In December 2024, officials announced a third invasive shothole borer species, exacerbating the crisis.
The toll is immense: thousands of trees removed, with symptoms like entry holes, wet staining, and sugary exudate signaling infection. In 2025, a hybrid symposium on Invasive Shothole Borers and Fusarium Dieback highlighted ongoing spread but no panic, emphasizing monitoring and sanitation. For residents, this means vigilant inspections and avoiding wood transport to prevent further outbreaks. California’s experience underscores how urban-agricultural interfaces accelerate invasive pests, costing millions in tree removal and replanting.
Australian Leaders Have Given Up Trying to Kill It. New Strategy is to Manage and Co-Exist.
Down under, the PSHB story took a sobering turn in 2025. Detected in Perth in 2021, the beetle prompted a national eradication effort, but by mid-2025, hopes dimmed. In June, the National Management Group (NMG), advised by experts, agreed eradication was no longer feasible due to widespread establishment. By November, they approved a Transition to Management (T2M) plan, shifting focus to containment and mitigation.
The Western Australian government, leading the response, allocated $6.9 million for a tree rebate program and $10 million in grants to support affected communities. This includes planting three new trees for every one removed, aiming to boost canopy coverage. Growers in the south-west brace for impact, with calls for better education to halt spread beyond metro areas. While disappointing, this pragmatic shift highlights the challenges of battling adaptable invasives in diverse climates, prioritizing sustainable coexistence over impossible elimination.
The Vulnerability of English Oaks and Other Oak Species
Oaks, symbols of strength and longevity, are tragically susceptible to PSHB. English oaks (Quercus robur) top the list of preferred hosts, often succumbing quickly as the beetle’s fungi ravage their vascular tissues. In South Africa, where PSHB invaded in 2017, English oaks and other exotics like Chinese maples have been decimated, altering urban treescapes forever.
Native oaks fare no better: California’s coast live oaks (Quercus agrifolia) show high infestation rates, with entrance holes increasing over time. Studies reveal a 7.5% annual rise in infestation probability, emphasizing oaks’ role in urban invasions. This vulnerability threatens biodiversity, as oaks support countless wildlife species. Homeowners with oak trees should monitor for frass (sawdust-like debris) and gumming, reporting suspicions promptly.
How Can We Product High Valued Trees?
There is good news. Preventive and therapeutic measures exist by vaccinating high valued trees with insecticides combined with fungicides. Research shows this duo reduces attack levels and sap flow issues in early infestations. A 2025 Perth trial saved over 100 infested trees via chemical injections, proving efficacy. Keep in mind that bark sprays are ineffective as they don’t reach the beetle inside.
For prevention, vaccinate trees before infestation, especially in at-risk areas. Be sure to contract with licensed applicators and integrate with cultural practices like pruning tool disinfection. While not foolproof, these treatments offer hope for preserving vulnerable species.
A Cautionary Tale for New Yorkers
Fortunately, as of December 2025, PSHB hasn’t reached New York State — no detections reported in searches or official records. But this beetle exemplifies how invasives disrupt local ecosystems. Like the emerald ash borer that’s already decimated ashes in the Northeast, PSHB is a threat on the horizon that has the potential to devastate our forests, rural, suburban and urban landscapes.
Invasives alter food webs, reduce biodiversity, and cost billions annually. PSHB’s global spread reminds us: buy local firewood, inspect plants, and support biosecurity. By learning from California and Australia, New York can stay vigilant and protect its forests.
In conclusion, the Polyphagous Shot Hole Borer is a wake-up call for proactive tree care. Whether you’re in an infested zone or not, awareness and action can stem the tide. Stay informed, report sightings, and let’s keep our trees thriving for generations.
